One of the principles guiding the design of the long-term evolution (LTE) system is transparency of the network to the user equipment (UE). For example, the UE is able to demodulate and decode its intended channels without specific knowledge of scheduling assignments for other UEs or network deployments.
For example, different downlink control information (DCI) messages on an enhanced physical downlink control channel (ePDCCH) may be transmitted from ports belonging to different transmission points. Even though there are several reasons for serving a UE with control signaling from different points, one application includes distributing parts of the scheduling algorithm at different points, such that, e.g., downlink (DL) transmissions are associated to a different point than uplink (UL) transmissions. In such a case, it makes sense to schedule DL and UL transmissions with control signaling provided directly from the respective points.
A further application includes serving a UE with parallel data transmissions from different points, e.g., for increasing data rate or during handover between points. A further application consists of transmitting system control information from a “master” point and relying on data transmission from other points, typically associated to pico nodes.
In all the above applications it makes sense to have the possibility to serve the UE with control signaling on ePDCCH from different points in the same subframe. In each case, UEs are not aware of the geographical location from which each reference signal (RS) port is transmitted.
Demodulation reference signal (DMRS) or UE specific RS are employed for demodulation of data channels and possibly certain control channels (e.g., ePDCCH). UE specific RS relieves the UE from having to know many of the properties of the transmission and thus allows flexible transmission schemes to be used form the network side. This is referred to as transmission transparency (with respect to the UE). A problem is however that the estimation accuracy of UE specific RS may not be good enough in some situations.
Geographical separation of RS ports implies that instantaneous channel coefficients from each port towards the UE are in general different. Furthermore, even the statistical properties of the channels for different ports and RS types may be significantly different. Example of such statistical properties include the received power for each port, the delay spread, the Doppler spread, the received timing (e.g., the timing of the first significant channel tap), the number of significant channel taps, the frequency shift. In LTE, nothing can be assumed about the properties of the channel corresponding to an antenna port based on the properties of the channel of another antenna port. This is an important aspect of maintaining transmission transparency.
Based on the above observations, the UE should perform independent estimation for each RS port of interest for each transmission. This results in occasionally inadequate channel estimation quality for certain RS ports, leading to undesirable link and system performance degradation.
In LTE, reference signals used for channel estimation are denoted as antenna ports. Hence, the UE can estimate the channel from one antenna port by using the associated reference signal (RS). One could then associate a certain data or control transmission with an antenna port (e.g., the UE may use the RS for that antenna port to estimate the channel used to demodulate the associated control or data channel). The data or control channel may be transmitted using that antenna port.
In LTE, the concept of quasi-co location has been introduced to improve the channel estimation performance when demodulating control or data channels. The UE may estimate long term channel properties from one reference signal in order to tune its channel estimation algorithm. For instance, the average channel delay spread can be estimated using one antenna port and used when demodulating a data channel transmitted using another antenna port. If this is allowed, it is specified that the first and second antenna port are quasi co-located (QCL) with respect to average channel delay spread.
Hence, as used in LTE specifications, two antenna ports are “quasi co-located” if the large-scale channel properties of the channel over which a symbol on one antenna port is conveyed can be inferred from the channel over which a symbol on the other antenna port is conveyed. The large-scale channel properties preferably include one or more of delay spread, Doppler spread, Doppler shift, average gain, and average delay.
In addition, or alternatively, the large-scale channel properties can include one or more of received power for each port, received timing (e.g., timing of a first significant channel tap), a number of significant channel taps, and frequency shift. By performing channel estimation algorithm tuning based on the RSs corresponding to the quasi co-located antenna ports, a quality of the channel estimation is substantially improved.
In current LTE specification, and according to working assumptions for new radio(s) (NR), a UE is not allowed to use measurements from one reference signal to assist in the reception or processing of another signal unless explicitly specified. The reason for this rule is that both the network and UE should have a common understanding on the relation between all reference signals and signals, to avoid situations where a UE is dependent on a relation between reference signals that the network might break without knowing.
As the UE is not allowed to make assumptions between reference signals that are not explicitly specified, it is not possible to allow UEs to improve the spatial receiver processing of one signal based on a previous reception of a previous signal.
The notion of quasi co-location (QCL) is introduced in LTE, and is under consideration for NR, to enable a way for a UE to use specific properties of one reference signal to assist in the processing of another signal. But QCL parameters for LTE only concern scalar entities that cannot directly be used for multi-dimensional spatial receiver processing.
The current QCL framework in LTE is designed for single input and single output channels in mind and lacks capability to take into account multi-antenna transmission, particularly for a large number of transmit and receive antennas. Moreover, how to handle high carrier frequencies and the use of beamforming together with QCL is a problem.